COPENHAGEN — The gathering risks of climate change are so profound that they could stall or even reverse generations of progress against poverty and hunger if greenhouse emissions continue at a runaway pace, according to a major new United Nations report.

Despite growing efforts in many countries to tackle the problem, the global situation is becoming more acute as developing countries join the West in burning huge amounts of fossil fuels, the Intergovernmental Panel on Climate Change said here on Sunday.

Failure to reduce emissions, the group of scientists and other experts found, could threaten society with food shortages, refugee crises, the flooding of major cities and entire island nations, mass extinction of plants and animals, and a climate so drastically altered it might become dangerous for people to work or play outside during the hottest times of the year.

“Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems,” the report found.

In the starkest language it has ever used, the expert panel made clear how far society remains from having any serious policy to limit global warming.

Doing so would require leaving the vast majority of the world’s reserves of fossil fuels in the ground or, alternatively, developing methods to capture and bury the emissions resulting from their use, the group said.

If governments are to meet their own stated goal of limiting the warming of the planet to no more than 3.6 degrees Fahrenheit, or 2 degrees Celsius, above the preindustrial level, they must restrict emissions from additional fossil-fuel burning to about 1 trillion tons of carbon dioxide, the panel said. At current growth rates, that budget is likely to be exhausted in something like 30 years, possibly less.